The present invention relates generally to wireless communications antennas. In particular, the invention relates to an improved feed network for using an array of radiating elements for more than one band of communications frequencies.
Dual band antennas for wireless voice and data communications are known. For example, common frequency bands for GSM services include GSM900 and GSM1800. GSM900 operates at 880-960 MHz. GSM1800 operates in the frequency range of 1710-1880 MHz. Antennas for communications in these bands of frequencies typically include an array of radiating elements connected by a feed network. For efficient transmission and reception of Radio Frequency (RF) signals, the dimensions of radiating elements are typically matched to the wavelength of the intended band of operation. Because the wavelength of the 900 MHz band is longer than the wavelength of the 1800 MHz band, the radiating elements for one band are typically not used for the other band. In this regard, dual band antennas have been developed which include different radiating elements for the two bands. See, for example, U.S. Pat. Nos. 6,295,028, 6,333,720, 7,238,101 and 7,405,710 the disclosures of which are incorporated by reference.
In some dual band'systems, wide band radiating elements are being developed. In such systems, there are at least two arrays of radiating elements, including one or more arrays of low band elements for low bands of operating frequencies (e.g., GSM900 and/or Digital Dividend at 790-862 MHz), and one or more arrays of high band radiating elements for high bands of operating frequencies (e.g., GSM1800 and/or UTMS at 1920 MHz-2170 MHz).
Known dual band antennas, while useful, may not be sufficient to accommodate future traffic demands Wireless data traffic is growing dramatically in various global markets. There are growing number of data service subscribers and increased traffic per subscriber. This is due, at least in part, to the growing popularity of “smart phones,” such as the iPhone, Android-based devices, and wireless modems. The increasing demand of wireless data is exceeding the capacity of the traditional two-band wireless communications networks. Accordingly, there are additional bands of frequencies which are being used for wireless communications. For example, LTE2.6 operates at 2.5-2.7 GHz and WiMax operates at 3.4-3.8 GHz.
One solution is to add additional antennas to a tower to operate at the LTE and higher frequencies. However, simply adding antennas poses issues with tower loading and site permitting/zoning regulations. Another solution is to provide a multiband antenna that includes at least one array of radiating elements for each frequency band. See, for example, U.S. Pat. Pub. No. 2012/0280878, the disclosure of which is incorporated by reference. However, multiband antennas may result an increase in antenna width to accommodate an increasing number of arrays of radiating elements. A wider antenna may not fit in an existing location or, if it may physically be mounted to an existing tower, the tower may not have been designed to accommodate the extra wind loading of a wider antenna. The replacement of a tower structure is an expense that cellular communications network operators would prefer to avoid when upgrading from a single band antenna to a dual band antenna. Also, zoning regulations can prevent of using bigger antennas in some areas.
Another attempted solution may be found in Application No. PCT EP2011/063191 to Hofmann, et al. Hofmann suggests using diplexers to combine a LTE frequency band at 2.6 GHz, with a SCDMA frequency band at 1.9-2.0 GHz, and applying both bands to a common radiating element. This helps reduce antenna width, but at a cost of increasing the number of coaxial transmission lines in the antenna. In the example of FIG. 2 of Hofmann, eight dual polarized radiating elements are illustrated per column. For each column, there would be eight LTE coaxial lines and eight SCDMA coaxial lines, for each of two polarizations, yielding a total of 32 coaxial lines per column. Given that there are four columns illustrated, the solution of Hofmann would require 128 coaxial lines just between the phase shifters and the diplexers.